This invention relates generally to nuclear reactors and more particularly to a system for transporting reactor coolant in nuclear reactors utilizing a liquid coolant.
One type of nuclear reactor presently in use produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors of this type, the heat produced thereby is used to generate electricity. Such nuclear reactors typically comprise one or more primary flow and heat transfer systems, and a corresponding number of secondary flow and heat transfer systems to which conventional steam turbines and electrical generators are coupled. A typical energy conversion process for such commercial nuclear reactor, therefore, involves transfer of heat from a nuclear core to a primary coolant flow system, to a secondary system which generates steam which is employed in the production of electricity.
In advanced liquid cooled nuclear reactors, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary system comprises a nuclear core within a reactor vessel, a circulating pump, a heat exchanger, and piping interconnecting the aforementioned apparatus. In nuclear reactors having more than one primary system, the nuclear core and the reactor pressure vessel are common to each of the primary systems.
The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the reactor core. The heated reactor coolant exits from the reactor vessel and flows to the circulating pump. In advanced breeder reactor designs such as this the reactor coolant then flows to the heat exchanger which transfers its heat to an intermediate flow system associated therewith. The cooled reactor coolant exits from the heat exchanger and flows into the pressure vessel, repeating the described flow cycle.
In addition to the reactor coolant found in the pressure vessel of liquid metal breeder reactors, the circulating pump generally has a reservoir of coolant above it in the pump enclosure. When there is no pump flow, the level of reactor coolant in the pressure vessel and the level of coolant in the pump reservoir are equal. When the pump is operating, however, a pressure differential occurs in the pipe between the outlet of the pressure vessel and the inlet of the pump. To maintain a constant pump flow, coolant from the pump reservoir must be utilized. The amount of pump reservoir coolant required is equivalent to the pressure differential in the pump suction pipe.
Because the pump impeller must always remain submerged, the amount of coolant in the pump reservoir must be adequate to compensate for any pressure differentials which may occur. A problem arises in that, if the pressure differential is great, the amount of reactor coolant in the pump reservoir becomes quite large. As the pump motor is external to the reservoir, the problem occurs in the length of shaft required between the propeller and the motor. There is a maximum length above which the length of shaft is not functional. This length limits the amount of reactor coolant which can be stored in the pump reservoir. This problem may be effectively solved by minimizing the amount of pressure differential in the pump suction pipe.
One means of minimizing this pressure differential would be to pressurize the cover gas found in the reactor pressure vessel of such advanced reactors above the reactor coolant level. This solution may pose a safety problem under certain conditions. In the unlikely event of a pipe rupture accident, the cover gas pressure would accelerate the discharge of the reactor coolant from the system. The rapid discharge of coolant is most undesirable under these conditions.
Another means to prevent excessive pressure differentials, and corresponding pump reservoir level drops, is to make the pump suction pipe quite large (e.g., 36 inches or more). This large pipe facilitates the flow of coolant to the inlet of the circulating pump. This method has its disadvantages. The large pipe size affects the size required for the containment buildings, the size of the primary system storage tanks, and requires the development of a large isolation valve for such large pipe. Additionally, because of the limited available pump suction head, the pump speed is limited to prevent cavitation. This requirement for low pump speed also affects the containment building size because low speed pumps are larger than high speed pumps.
The optimum solution to the problem of eliminating the pressure differentials found in the pump suction pipe is one which does not cause any safety problems, and which permits reduction in size, and the associated costs, of the primary containment building.